As is well known to one skilled in the field of aeronautics, there are constraints as to the emissions from aircraft turbine power plants. For example, the United States Government imposes restrictions on the noxious fumes and smoke that are emitted from the engine. Obviously, there is an on-going effort from scientists and engineers to lower the levels of smoke and noxious constituents release in the combustion process. Needless to say it is imperative that the reduction of the smoke levels cannot adversely impact the efficiency of the engine. Hence, to effectuate reduction of smoke it is necessary to increase combustor swirler airflow without sacrificing diffuser stability and performance which would otherwise impact the efficaciousness of the engine. Additionally, as one skilled in this art appreciates, it is abundantly important that the pressure drop across these swirlers are maintained at a minimum as this has an impact on cycle energy and the higher the pressure drop the greater the deficit. Obviously, the designer is confronted with the problem of needing to increase the airflow through the swirlers while at the same time to maintain a specified pressure drop. While this suggests increasing the opening of the swirler to maximize the flow of air, a simple increase in flow area reduces the velocity of the air, thereby adversely affecting fuel atomization and fuel-air mixing in the primary zone of the combustor.
For a good understanding of this invention it is believed that an understanding of the problem would be of some value. As mentioned in the earlier paragraphs, there is a need to increase combustor swirler airflow without sacrificing diffuser stability and performance. As is appreciated by those skilled in this technology, typical current day designs of the swirlers that are utilized by the fuel nozzle are high shear swirler systems that rely on being fed from the cowl of the combustor which receives air from the compressor. At the swirler inlet, the pressure of the swirler-feed air is cowl static pressure, rather than the higher, total pressure of the cowl-entry air, because the swirler inlet is perpendicular to the cowl-entry flow. The high dynamic head in the cowl inlet-flow is lost primarily because very little diffusion of the cowl flow occurs between the cowl opening and the swirler inlet. By performing flow visualization tests in a sector cowl flow rig, it could be seen that the high velocity air entering the cowl stagnates on the swirler bearing plate and dome between swirlers. These visualizations and pressure measurements indicate that a significantly depressed swirler feed pressure in the area of the adjacent swirler just downstream of the cowl inlet as compared to the feed pressure in the area away from the cowl inlet, which is cowl static pressure fed. This depressed swirler feed pressure in combination with the pressure loss of the cowl flow stagnating on the bearing plate lead to reduced cowl pressure recovery, and consequently reduced swirler airflow.
We have found that we can provide an aerodynamically efficient solution to the problems stated above by incorporating a bearing plate that is mounted on the fore end of the fuel nozzle and contoured to provide an aerodynamically smooth flow to a scoop mounted on the fore end and at the inlet of the radial air swirler. The combined bearing plate and scoop serve to define an aerodynamically clean passage for leading an increased amount of air flow into the inlets of the swirler with a consequential reduction in smoke promotion without sacrificing diffuser stability and without increasing the overall combustor pressure drop. By virtue of this invention, the swirler airflow is increased and tests have confirmed that as much as a 2.5% increase has been realized with the additional benefit of providing a circumferentially uniform swirler feed pressure. One important aspect of this invention is that the benefits described above can be obtained with a small modification to existing swirler and fuel nozzles. The existing swirler casting can be modified to include the scoop configuration and the bearing plate can easily be configured to include the aerodynamic design of this invention.